Aluminium is produced conventionally by the Hall-Heroult process, by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperatures up to around 950.degree. C. In Hall-Heroult cells, the anodes are usually pre-baked carbon blocks that are consumed by the electrochemical reaction, corroded by contact with the electrolyte and disintegrated by the air and/or oxidizing gases present. Soderberg anodes made of a coherent carbon mass which solidifies in situ are also used.
Pre-baked anodes for aluminium production are made of a matrix of petroleum coke with pitch as binder. Their production involves various phases including preparing and treating the starting materials, mixing, forming and calcining at high temperature, followed by securing the current supply member by rodding.
The resistance of that part of the anode which remains outside the bath during cell operation is of paramount importance, not only to decrease the amount of anode consumption above the theoretical requirement but also to reduce the formation of carbon dust. It is advantageous to reduce carbon dust, for it causes of a reduction in current efficiency and an increase in cell temperature, and must be eliminated when it collects on the bath surface.
Of the several attempts to protect the anode, none has so far been entirely satisfactory. The normal protection by aluminium spraying is costly and not always impervious. The oxidation of the carbon anodes, in the Hall-Heroult cell, outside the bath leads to a loss for the aluminium producer. Typically, instead of the theoretical consumption of 0.33 kg of carbon per ton of aluminium, often more than 0.43 kg is lost, the difference being caused mainly by air and CO.sub.2 burn.
Many elements or compounds catalyze the oxidation reaction of carbons but the inhibition of the oxidation reaction is more difficult to achieve. In general, the oxidation reactivity of carbon is reduced with absorbers, or with ceramic protection layers. Several absorber additives have been reported, such as metal, halogen compounds, and incorporated nitrogen. Ceramic protecting layers have been proposed, formed by low melting liquid glass, such as B.sub.2 O.sub.3, Cr.sub.2 O.sub.3, silica, etc. See, e.g., French patent no. 1,107,113 (1955) and U.K. patent no. 760,623.
The oxidation prevention treatment processes contemplated for the anode can be divided into two different groups, one is an additive added after the anode baking, the other is an additive added into the carbon paste. Until recently, only an aluminium coating protection treatment, or a thick layer of alumina and cryolite, has worked reasonably well for oxidation protection of commercial pre-baked anodes; however, these have several drawbacks, such as cost and difficulties in the cell operation. Several other oxidation prevention treatments have worked well in the laboratory but have fallen short of the expected performance when the same treatments have been applied to the anodes tested in commercial cells. No apparent reason has been forthcoming, and the discussion of such an effect has invariably been directed towards the possibility of the composition of the anode gases being the reason for such a difference.
When boron has been added to the anode paste in the form of elemental boron or boron compound, the oxidation rate of the carbon has been reduced but the consistent contamination of aluminium is usually unacceptable.
Recently, U.S. Pat. No. 5,486,278 (Manganiello et al.) has disclosed a treatment process which has been shown to significantly reduce the oxidation of the anode in the laboratory as well in commercial cell tests of pre-baked carbon anodes. This method comprises treating the anode or other component in a boron-containing liquid to intake the boron-containing liquid to a selected depth over parts of the surface to be protected. This selected depth is generally in the range of from 1 to 10 cm, preferably at least 1.5 cm and at most about 5 cm, preferably still at least about 2 cm and at most about 4 cm. This method was found to significantly reduce the oxidation of pre-baked anodes in laboratory tests and in commercial test cells. It was found unexpectedly that the greatly improved oxidation resistance obtained with this treatment is partly offset by a strength loss which could lead to burn-off after a critical weight loss when the anode is subjected to stress.
An article entitled "The Reactivity of Carbon Electrodes and Its Dependence on Organic Catalyst Inhibitors" by Gosta Wranglen from Jernkont. Ann. 142 (1958):10, recognizes that oxidation resistance in carbon materials can be increased by adding, or impregnating or coating such materials with certain phosphates of zirconium, by impregnating with fused aluminum and by impregnating or coating with siliciferous compounds, such as treatment liquids including approximately 10 weight percent of substances such as: Al.sub.2 O.sub.3, CaF.sub.2, CaB.sub.4 O.sub.7, B.sub.2 O.sub.3 as H.sub.5 BO.sub.3, P.sub.2 O.sub.5 as (NH.sub.4).sub.2 HPO.sub.4 and V.sub.2 O.sub.5 as NH.sub.4 VO.sub.3, in water. While the Wranglen article does mention the possibility of boron contamination, it does not suggest any way of taking advantage of the properties provided by impregnation with boron while avoiding contamination of the product aluminium due to excessive boron. In addition, the Wranglen article throughout recommends a minimum of at least 0.5 weight percent boron compound in the entire carbon substrate. Such a high content, is not permissible.
More recently, in U.S. patent application Ser. No. 08/584,047 (Sekhar et al.), filed Jan. 10, 1996, and in International application PCT/US97/00304, filed Jan. 10, 1997 (Selkar et al.), (together referred to as "the high strength applications"), there has been disclosed a treating liquid containing at least one soluble boron compound and at least one additive from the group consisting of aluminium compounds, calcium compounds, sodium compounds, magnesium compounds, silicon compounds, elemental carbon, and elemental aluminium, the additive being in the form of a powder, in suspension, as a colloid, or in solution at 80.degree. to 120.degree. C. A preferred formulation, contains, per 100 ml of water (including a small quantity of a surface-active agent), 2-10 grams of boron (as metal in the form of a boron compound) and as additives, aluminium acetate boric together with at least one of calcium acetate and calcium carbonate (the total amount of additives not exceeding the amount of boron). Treatment by this treating liquid avoids the aforementioned strength loss problems. As described in the high strength applications, even with the highest achievable levels of boron concentration, the problem of process contamination is avoided because the protective boron compounds are present only in the surfaces needing protection, and only to a depth of a few centimeters or less. International applications PCT/US97/01080 (Sekhar et al.), filed Jan. 27, 1997 and PCT/US97/02041 (Sekhar et al.), filed Feb. 7, 1997 addressed this concern by proposing a lower boron treating liquid ("the low boron applications"), which will provide comparable oxidation resistance and strength properties as provided by the treating liquids described in the high strength applications. The treating liquids of the low boron applications contains a borate of an alkali metal or of an alkali earth metal, such as a diborate, metaborate, tetraborate and the like. Preferably, sodium tetraborate (such as Na.sub.2 B.sub.4 O.sub.7.10H.sub.2 O) is used. More preferably, at least one nuclei-forming compound selected from aluminium acetate boric (such as CH.sub.3 CO.sub.2.Al(OH).sub.2.1/3H.sub.3 BO.sub.3), aluminum fluoride (such as AlF.sub.3.3H.sub.2 0), gibbsite (AM(OH).sub.3) or aluminium nitrate (such as AlNO.sub.3.9H.sub.2 O) is used in combination with the borate of sodium. As used herein, a "nucleus" is defined as a tiny particle of solid that forms from a liquid as atoms cluster together; because these particles are large enough to be stable, nucleation has occurred and growth of the solid can begin. Preferably, the low boron treating liquid is an aqueous solution.